1. Field of the Disclosure
The present disclosure relates to a beam forming device and a corresponding method. The present disclosure relates further to an active imaging device and an active imaging method for imaging a scene as well as to a beam forming unit and a processing method. Still further, the present disclosure relates to a computer program and to a computer readable non-transitory medium storing such a computer program. The present disclosure relates particularly to MIMO beam forming devices and methods.
2. Description of Related Art
Active imaging systems are becoming more and more popular at ultrasonic, microwave, millimeter and terahertz frequencies for a number of applications including medical and security applications.
The arrangement of transmitter (herein also called “transmit element”) and receiver (herein also called “receive element”) in an active imaging system may take on many different forms. In an embodiment relevant for the present disclosure multiple transmitters and receivers work together to form a MIMO radar (or MIMO active imaging system.) There are predominately two different types of MIMO radars. The first type is called statistical MIMO, in which the antennas (generally the “transmit elements” and the “receive elements”) are placed far apart from each other to provide different views of the object (generally the “scene”). The second type of MIMO is called beam forming (or co-located) MIMO in which the antennas are placed close to each and act together to form a “virtual” beam forming array. The present disclosure primarily applies to the beam forming MIMO arrangement.
MIMO beam forming in one dimension is typically combined with other techniques (i.e. synthetic aperture radar) to form a 2D image. Alternatively, MIMO beam forming can be performed in two dimensions to form a 2D image. To yield a full 3D image of an object (or a 2D image with additional distance/depth information), such arrangements typically transmit a wideband continuous waveform (i.e. frequency modulated continuous wave (FMCW)) or a wideband pulse to provide ranging information.
For any of the above MIMO beam forming arrangements, when there is a large distance between the object and the receiver/transmitter arrangement (the so called “far field” case) the optimum weights for a given angular direction to perform MIMO beam forming do not change significantly when the distance to the object is varied. However, when the distance between the object and the receiver/transmitter arrangement is short (the so called “near field” case) the optimum weights to perform MIMO beam forming change significantly with changing distance.
Imaging systems using the MIMO beam forming technique and obtaining distance information are generally known.
J. H. G. Ender, J. Klare, “System Architectures and Algorithms for Radar Imaging by MIMO-SAR”, IEEE Radar Conference 2009 describes a system in which a MIMO beam forming array is used in one dimension (in this case in the direction of the aircraft wings) and a Synthetic Aperture Radar (SAR) is created in the movement direction (in this case the aircraft's forward motion) to create a 2D image. The third dimension (distance from the aircraft to objects on the ground) is obtained by using the frequency modulated continuous wave (FMCW) technique. However, this system operates in scenarios where there is a large distance between the transceiver and the objects on the ground.
J. Klare, O Saalmann, H. Wilden, “First Experimental Results with the imaging MIMO Radar MIRA-CLE X”, EUSAR Conference 2010 describes a system in which SAR is combined with MIMO beam forming to create a 2D image and FMCW is used to provide the third dimension, which is the distance information to the object. This system is used in a similar way to the system described by J. H. G. Ender et al., in which the distance between transceiver and receiver is very large.
X. Zhuge, A, Yaravoy, “MIMO-SAR Based UWB Imaging for concealed Weapon Detection”, EUSAR conference 2010 Aachen, Germany, p. 195-197 describes a system in which SAR is combined with MIMO beam forming to create a 2D image. The third dimension (distance to the object) is obtained by using a pulse based ultra wideband signal (UWB).
X. Zhuge, A. Yarovoy, “Near-Field Ultra-wideband Imaging with Two dimensional Sparse MIMO Array”, Proceedings of the fourth European Conference Antennas and Propagation (EuCAP) 2010, p. 1-4 describes a system in which the MIMO beam forming in used in two dimensions to create a 2D image. The third dimension (distance to object) is obtained by using a pulse based ultra wideband signal (UWB).
The “background” description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description which may not otherwise qualify as prior art at the time of filing, are neither expressly or impliedly admitted as prior art against the present invention.